This invention relates to a method of compensating a density log.
It is well known to log boreholes using a density logging technique.
In this technique a tool containing a source of gamma radiation is inserted into a borehole. The radiation penetrates the surrounding formation, where it collides with atomic electrons in the formation. According to the well-known Compton effect this results in the backscattering of a certain amount of gamma radiation, some of which returns to the tool.
The tool contains a pair of radiation detectors (i.e. scintillation crystals). One of these is commonly termed the “near detector” and lies closer to the gamma source than the other. The latter is termed the “far detector”.
The near detector provides data on the density of a region in the vicinity of the borehole wall. The far detector provides such data with regard to locations spaced from the borehole wall.
It is known that mudcake formed on the interior of the borehole wall, and drilling mud (or other fluid) in the borehole column, cause inaccuracies in the far detector measurements. In the absence of the near detector values therefore it would be impossible to obtain accurate density measurements. However the use of both the near and far detectors allows the use of the signals from the former to compensate the data from the latter for the effects of mudcake and column mud/other fluid.
The paper “The Dual-Spaced Density Log—Characteristics, Calibration, and Compensation” (The Log Analyst, January-February 1992, pp. 42-49) sets out inter alia a method of using dual (ie. near and far) detectors to compensate backscattered gamma density readings for the effects of mudcake and column mud or other fluid in the annular space between a logging tool and a borehole wall caused by the stand off of the tool.
The attention of the reader is directed to the aforementioned paper, the entire disclosure of which is incorporated herein by reference.
In essence the technique disclosed in “The Dual Spaced Density Log—Characteristics, Calibration, and Compensation” involves defining a geometric factor G, which describes the depth of measurement penetration of each of the detectors in the logging tool.
G is defined asG=1−e−kr in which k is a constant; and r the radial distance from the tool.According to the geometric-factor theory on which the compensation in the “The Dual Spaced Density Log—Characteristics, Calibration, and Compensation” is based, the geometric factor Gm attributable to the mudcake, and the geometric factor Gf attributable to the formation fluid, sum to unity. Substituting various known expressions for the terms in G allows the derivation of a compensated formation density expression.
Although the technique of “The Dual Spaced Density Log—Characteristics, Calibration, and Compensation” has proved highly successful, it relies on a number of assumptions which are not applicable to every well under investigation.
Firstly, the method assumes that the tool is stood-off parallel to the formation, by reason of the mudcake being of constant cross-section and parallel sided.
In caved-in or rugose wells this is unlikely to be the case.
Some other wells suffer from drilling-induced cyclic “spiralling” the nature of which will be known to those of skill in the art. Such spiralling also renders inapplicable the “parallel standoff” assumption in “Dual Spaced Density Log—Characteristics, Calibration, and Compensation”.
Additionally, the statistical noise on any radiation counting system is related to the square root of the counting rate. Most of this noise arises from the long-spaced detector in a dual-spaced detector system because this detector has a relatively low counter rate due to its distance from the radioactive source.
The compensation technique of “The Dual Spaced Density Log—Characteristics, Calibration, and Compensation” is successful if the interfering effect is synchronous, or common, to the two detectors, as arises from mudcake. If the interfering effect however is not synchronous (such as derives from counting statistics) it does not cancel, and simply appears as (undesirable) noise in the processed (final) log.
This undesirable noise is further exacerbated by the known compensation process that combines the measurements of the near and far density detectors together, typically, but not exclusively, in a linear way. However this is done, the result is to multiply the measurement from the far detector by a number greater than unity. Typically, this combination can be represented by the equation:Compensated density=A*Far density+(1−A)*Near densityWhere A is a constant that is greater than 1